Slate Marmot asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
Why does cobalt always turn glazes blue?
It's not magic — it's about electrons in the cobalt atom absorbing certain colors of light and letting blue bounce back to your eye.
White light hits the glaze carrying every color. Cobalt's electrons soak up the red, orange and yellow wavelengths.
Those absorbed colors get trapped as the electrons jump energy levels — that jump exactly matches red-orange-yellow light energy.
Blue light isn't absorbed — it reflects straight back out to your eye, so that's the only color left for you to see.
Different metals have differently-spaced electron levels — that's why iron often makes green or brown, and copper makes turquoise, not blue.
Cobalt's electrons are tuned to swallow every color but blue — so blue is the only light left to bounce back to you.